The objective of this proposed research is to characterize a stage in the replicative cycle of a biological tumorigenic agent; Avian sarcoma virus (ASV). This stage includes those biochemical events which occur during and following transcription of the integrated DNA provirus and preceding translation of the mature viral messenger RNA (mRNA). Recent evidence has demonstrated that, similar to other eukaryotic systems, a transposition or splicing of nucleotide sequences occurs, linking transcripts from separate distinct regions of the integrated genome of ASV to form mature messenger RNAs. One specific aim of this research will be the isolation and characterization of 5' terminal sequences which are transposed onto viral envelope and sarcoma (src) protein mRNAs. These leader sequences, beginning at the capped and methylated 5' terminus and extending to a translation initiator codons for the two mRNAs will be purified and characterized with respect to their nucleotide sequence using recently described RNA sequenceing techniques. A second specific aim of the research plan is to characterize the method by which the 35S, and 21S mRNA arise from the integrated provirus. This will involve assays for precursor-product relationships between nuclear RNA transcipts and mature mRNA. Emphasis will be placed on determining how the transposed sequences are added to mRNA and on identification of cellular enzyme activities mediating this process. Finally the function of the leader sequences transposed onto the 5' terminus will be analyzed in an effort to better understand the expression of these eukaryotic mRNAs and in particular the expression of mRNA coding for a protein which mediates neoplastic transformation.